1. Field of the Invention
The present invention relates to producing upgraded calcium tungstate concentrates from solid, impure calcium tungstate-bearing material such as concentrates, and also relates to the production of a calcium tungstate concentrate from a sodium tungstate liquor such as a leach liquor produced by digestion of calcium tungstate-bearing material with aqueous sodium carbonate.
Calcium tungstate, CaWO.sub.4, is one form in which tungsten is conventionally sold to consumers of tungsten. Typically, commercially acceptable calcium tungstate contains at least about 65 wt. % tungsten as WO.sub.3, and contains no more than minor amounts of impurities such as copper, sulfur, phosphorus, bismuth and molybdenum.
Although calcium tungstate (as scheelite) is one of the major naturally occurring forms of tungsten, the calcium tungstate as found in nature is contaminated with gangue and impurities which greatly reduce the WO.sub.3 content of the raw material. The gangue and impurities must be removed for the calcium tungstate to be a useful product. Concentration techniques such as froth flotation and gravity concentration are generally effective to separate the calcium tungstate from associated gangue, but some of the tungsten concentrates thus produced still contain only about 10 wt. % to 20 wt. % WO.sub.3. Thus, it is still necessary to treat the concentrates to raise the WO.sub.3 content. In addition, alkaline-based processes for the recovery of tungsten frequently generate an alkaline solution of sodium tungstate, and it is desirable to be able to treat such a solution to recover an upgraded calcium tungstate concentrate. There is still a need for an economical, efficient process for producing upgraded calcium tungstate concentrates, which should advantageously generate by-product or side streams which are themselves useful in the purification of tungsten.
2. Description of the Prior Art
One process that has been employed for upgrading calcium tungstate concentrates comprises adding hydrochloric acid to the concentrate to dissolve calcite (CaCO.sub.3) and thereby raise the WO.sub.3 content relative to the remaining solid material. This process is unsatisfactory because it consumes excessive quantities of expensive acid, and dissolves a portion of the tungstate from the feed material, necessitating an extra series of tungsten recovery steps to maintain the overall tungsten recovery at reasonable levels.
The conventional method to produce synthetic scheelite from the pregnant liquor generated by soda ash pressure leaching of off-grade scheelite concentrates involves neutralization of the excess sodium carbonate with H.sub.2 SO.sub.4. Carbon dioxide is evolved, and sodium sulfate generated. Calcium chloride is then added to form a calcium tungstate precipitate plus sodium chloride waste liquor. This neutralization with H.sub.2 SO.sub.4 is necessary to avoid precipitating calcium carbonate when the calcium chloride is added. Sulfuric acid is consumed, and excess sodium carbonate is converted to sodium sulfate instead of being recycled to the leaching operation. Furthermore, the presence of this substantial concentration of aqueous sodium sulfate results in coprecipitation of calcium sulfate. Sulfur contamination of the calcium tungstate precipitate results. Therefore, the precipitate is calcined with carbon at about 950.degree. C. to reduce the calcium sulfate to calcium oxide plus SO.sub.2 offgas. The contaminating sulfur thus leaves as SO.sub.2.
This process is unsatisfactory because it consumes large quantities of sulfuric acid and calcium chloride, and because it generates large volumes of waste sodium sulfate and sodium chloride in solution which must be disposed of. Furthermore, it would be desirable to eliminate the calcining operation, with its associated sulfur oxide evolution.
As will be seen below, the process of the present invention is distinct from both of the foregoing processes. It utilizes the reverse of the typical alkaline leaching reaction under conditions of temperature and pressure such that calcium tungstate, rather than being consumed as might be expected, is produced.